The need in industry for vibration isolation is growing. For example, there is less and less tolerance for environmental vibration in ultraviolet steppers used in semiconductor manufacturing. As the manufacturing of semiconductors and other products becomes more and more precise, the need for suppressing environmental vibration becomes greater and greater.
Workers in the art have conceived of a theoretical active vibration isolation system which uses force motors, such as voice coil motor elements, and sensors on the isolated payload to measure the absolute motion of the payload relative to inertial space. So far, these prior art conceptions and arrangements have not been fully practical because the coupling of payload resonances with sensed outputs compromise stability margins.
In the simplest possible piezoelectric active vibration isolation system, a system resonance frequency is formed by the spring stiffness of the piezoelectric motor elements in combination with a supported payload mass. Typical supported payload weights are in the range of 1000 pounds per piezoelectric motor. A typical piezoelectric motor element will have a spring stiffness coefficient of approximately 1.5 million pounds per inch. This gives a troublesome system resonance frequency of about 130 cycles per second. The value of this system resonance frequency (relative to the frequency range in which isolation is required) leads to two problems which must be alleviated to obtain a practical active isolation design. The first problem is that the system feedback loop gain must be quite high to obtain active vibration isolation down to approximately 1 hertz. Further, the gain must be filtered to lower the gain below unity at the payload/motor resonance frequency to ensure stability. In prior art designs, this desired gain has been impossible to obtain. Second, the system greatly amplifies environmental vibration at the payload/motor resonance frequency and since feedback gain at this frequency is low, most of the benefit of the active isolation system is lost in such a design. The need therefore continues to exist for a practicable active vibration isolation system based on piezoelectric motors or other stiff actuators.